Con Ed customers in the five boroughs can expect an average $9 monthly jolt to their electric bills starting in January, thanks to a price boost quietly granted by the Federal Energy Regulatory Commission.

Shocked city and state officials yesterday begged FERC to reconsider its New Year’s gift to power companies.

“Customer bills will increase by hundreds of millions of dollars,” the officials complained in papers filed with the agency.

FERC took heat last year for a ruling that would have let the same generating companies charge consumers $500 million for city property taxes that the companies never had to pay.

FERC did a U-turn after a Post campaign and lobbying by the city and the state.

If it stands, FERC’s latest ruling will cost power users across the state $400 million to $500 million, the New York Power Authority estimates. Con Ed’s Big Apple customers, both individuals and businesses, would be stuck with between $300 million and $375 million of that bill.

The price boost is an “unwarranted transfer of wealth from local and state governments and consumers” to power-plant owners, said Power Authority President Gil Quiniones.

The Port Authority, the MTA and the state Public Service Commission are also fighting the increase.

Con Ed also complained about the price boost, which it would be forced to pass on to its customers.

The griping is nonsense, contends US Power Generating Co., which owns one of the plants that will benefit from FERC’s rules — which cover auctions where firms that distribute electricity, like Con Ed, purchase power from companies that generate it.

US Power spokesman John Reese disputes officials’ complaints that the rules will raise power prices by up to $500 million. “I have no idea where they came up with a number like that,” he said.

Whatever extra cash generating companies gain will ensure that there is money to maintain and operate power plants, he said.

FERC granted the boost a month ago in a jargon-laden, 55-page ruling.

The issue arose because of a 550-megawatt power plant, Astoria Energy II, which came on line in 2011.

The Power Authority contracted to buy all the electricity produced by the facility, which operates more cheaply than some competitors.

US Power says that Astoris Energy II’s lower costs would amount to unfair competition without auction rules that would allow its competitors to charge higher prices.